The lithography process is one of the most important procedures of the integrated circuit manufacturing technology, and mask as an article which is indispensable in lithography has a huge effect on the lithography process. The base material used to fabricate the mask is a photomask substrate. And the masks provided by different mask providers will be made by using different photomask substrates provided by different providers, or by using different batches of the photomask substrates provided by the same provider. The quality of the mask received by the lithography will be different due to the difference in the photomask substrate and the difference in the fabrication process of the mask, thereby resulting in that there is difference between the optimum process conditions of different products.
During the lithography, it is required to set process conditions including a defocus amount and an exposure amount, and when the defocus amount and the exposure amount are within a certain range, the lithography process can provide the desired linewidth and overlay error. The lithography engineer needs to ensure all graphs on the mask with appropriate process conditions. To determine the optimum process condition for each of the masks, it is usually to make a focal distance-energy matrix for each of the masks to find the optimum exposure energy and the focal distance, and perform a process window analysis to finally find the most appropriate defocus amount and exposure range. The focal distance-energy matrix indicates a test pattern in two-dimensional distribution of difference exposure energy and focal distance in one silicon wafer.
As shown in FIG. 1, the different exposure amount setting is adopted in each of different regions in a horizontal direction, and the different defocus amount setting is adopted in each of different regions in a vertical direction, so as to realize to adopt different exposure amount setting and/or different defocus amount setting in each region and to finally form the same image in each region by the lithography and measure the image formed in each region, thereby obtaining CD value of the image formed in different regions on the wafer and the whole data set of the corresponding exposure amount and defocus amount, and finally determining the optimum defocus amount and exposure amount according to the obtained optimum CD value of the image.
In the existing lithography process, it is usually to determine the optimum lithography process condition by making a focal distance-energy matrix for each of the masks in the current layer. This action has such problem that it is required to spend extra time and labor each time when the new process condition is determined. To reduce the time spent in determining the new process condition, the lithography engineer will pre-estimate the energy according to the size error of the linewidth of the mask, however such prediction method can't pre-estimate the focal distance; alternatively, the lithography engineer pre-estimates the process condition only from personal experience, however such pre-estimation has no theoretical supports, therefore there is necessary a certain subjective error.
At present, the demand for the producing new products becomes faster and faster, and one problem to which we face is how to decrease the trial production time for determining the process condition and ensure the accuracy of the process condition when the new products are introduced.